The present invention relates generally to a sheet guiding device for guiding sheets between units of a printing machine, and more particularly, to an air assisted sheet guiding device.
EP 0 156 173 B1 discloses a sheet guide device or arrangement for guiding sheet material that is printed on one or on both sides. This guide arrangement is formed by modularly arranged flow channels which have openings that define air nozzles in a guide surface. The flow channels have a plurality of fans for supplying positive pressure (blowing or blast) air or suction air.
DE 42 44 499 C2 also discloses a sheet guide arrangement with openings for the passage of air. A positive pressure (blast) or suction box presents guide or cover surfaces connected by means or blast/suction air connecting members with a blast-air or a suction-air source. From the cover surface, flow resistance-forming bodies extend into the blast and suction air box. With increasing distance from the orifice zone of the blast or suction connecting member the flow resistance state of the above-mentioned bodies in the blast or suction box decreases. Here, it proves disadvantageous especially in blast-air operation, with this arrangement of the flow resistance-forming bodies because turbulence arises in the region of the passage openings, which can lead to unstable passage of the sheets.
An object of the present invention is to provide a sheet guide arrangement in a printing machine which eliminates the foregoing disadvantages, which enables secure and stable sheet guidance, especially in the blast-air operation, as well as also in suction-air operation, and which reduces the creation of turbulence.
A first advantage of the invention lies in that the sheet-conducting print material can be uniformly and reliably guided free from smudging in the blast-air as well as in the suction-air operation of the sheet guide arrangement. By means of the inventive construction of the sheet guide arrangement, turbulence is avoided in the region of the air openings, so that in the blast-air operation a stable air cushion with substantially uniform pressure distribution emerges from openings in the guide surface and develops between the guide surface of the sheet guide device and one side of the sheet-form print material. The danger of smearing is thereby reduced, since the air flow, which preferably diffuses an air cushions guides the print material with essentially uniform pressure distribution.
It is advantageous, furthermore, that the sheet guide arrangement can also be operated in suction air operation. In that case, the side of the print material, which preferably is unprinted, is uniformly drawn in the direction of the guide surface of the sheet guide arrangement.
The air supply for the sheet guide arrangement preferably is a reversible regulatable pneumatic system, which permits a positive pressure or suction-air supply. Alternatively, reversibly operable, variable speed fans may be used for directing or drawing the air.
The guide sheet arrangement is located in the printing machine adjacent the sheet transport systemsxe2x80x94which in this_case includes gripper bridges-on the sheet guide cylinders (i.e., contact drum, transfer cylinder, printing cylinder, and turning system) at a defined spacing, whether it be straight or curved. Moreover, the sheet guide arrangement can be positioned underneath, as well as above sheet-conducting cylinders, drums, or circulating conveyor systems.
A further advantage of the sheet guide arrangement is that by virtue of directional elements the air flow stream and pressure distribution can be controlled over the sheet-conducting surface. The air introduced into a flow channel from the particular pneumatic system can be controlled in a uniform manner with uniform pressure distribution inside the flow channel, which enables more stable support for the passing sheets. By an air-permeable construction, for example a perforation of the directional elements, possible recirculation of the air flow is eliminated and a stabilizing of the flow in the flow channel is achieved. By means of an inclined arrangement of the directional elements (or portions thereof), air may be directed into or drawn from the flow channel at a predetermined angle by the pneumatic system, and by use of air permeable materials such as perforated materials with portions or corners cut away the air flow in the channel may be reduced. This likewise can lead to a uniform pressure distribution of the flow channel.
The guiding surface of the sheet guide arrangement may be formed as an air-permeable guide surface with passage openings for transverse air flow. By means of such air-permeable guide surface diffused air flows can be generated that act upon the underside and/or the upper side of a sheet-form printing material, in the form of blast-air or suction-air flows.